Liquid tanks, particularly fuel tanks for motor vehicles, are currently generally fitted, amongst other things, with a venting circuit. This circuit allows air to be introduced into the tank in the event of underpressure (in particular to compensate for the volume of liquid consumed) or allows gases contained in the tank to be removed in the event of overpressure (particularly in the event of overheating). This circuit also allows the gases that have to be discharged into the atmosphere to be routed and possibly filtered for the purpose of meeting the increasingly strict environmental requirements in this area.
The venting circuit includes, in a known manner, at least one valve that prevents, as far as possible, liquid from escaping the tank in the event of the tank being turned upside down or at an excessively high tilt angle. This venting valve must provide a rapid and reliable response when its operating conditions arise, but with minimal sensitivity to transient phenomena such as in particular very high flow rates, overpressure in the tank or low-amplitude waves. It must also ensure that there is minimal liquid carried over into the canister (or the chamber containing a substance, usually activated carbon, which adsorbs the fuel vapors) in normal operation and when filling, in order to avoid saturating said canister and making the decontamination of the gases discharged into the atmosphere ineffective. This phenomenon is generally called LCO (liquid carry over) in the jargon of the field.
Many venting valves employ a float having an upper needle or tip which closes off an aperture for connecting the tank to the venting circuit (known as the ventilation aperture). One way of reducing the risk of LCO with this type of valve is that described in Application WO 2006/125758 in the name of the Applicant, the content of which is incorporated by reference in the present application, and which consists in providing the valve with baffles (preferably, at least one internal and one external baffle) so as to create a chicane or tortuous path for the vapor stream. For this purpose the baffles and the housing of the valve are provided with openings in their upper part and preferably the openings of the internal and external baffles are aligned and arranged crosswise relative to those of the main housing. This geometry prevents direct flow between the various partitions and therefore creates an optimal labyrinth effect.
Another way of solving this problem, which is described in Application WO 2008/028894 also in the name of the Applicant, and the content of which is also incorporated by reference in the present application, consists in providing the lateral surface of the head of the float with a baffle and in adapting the internal geometry of the housing and of the head of the float so that the vapor streams from the tank impact on this baffle before going through the ventilation aperture.
Although effective, these two solutions have the major drawback of complicating the geometry of the valve and therefore of increasing the costs linked to its manufacture. Moreover, they both focus on the geometry of the pathway for the vapors in the upper part of the valve, between the ventilation openings (openings in the housing of the valve) and the ventilation aperture.
However, liquid fuel which has entered into the valve through these ventilation openings and which is lead down to the base of the valve by the aforementioned devices, must be able to be drained/purged to prevent liquid fuel from accumulating in the valve. Moreover, fuel must also be able to penetrate into (and be discharged from) the valve, preferably through its bottom portion since its operating principle is precisely linked to the fact that the float must be able to respectively open/close the ventilation aperture depending on the level of fuel in the tank.
Hence, apertures known as drainage apertures are generally positioned at a low point of the valve, often in the base thereof. In order to optimize the purge operation, it is advisable to increase the size of these apertures. But by doing this, there is a risk of creating a direct path for the liquid fuel which can therefore very rapidly rise into the housing, along the float and reach the ventilation aperture before the float has effectively blocked the ventilation aperture. This situation also leads to LCO.
If the drainage apertures are smaller, said apertures may no longer fulfill their drainage role. If the liquid fuel is not correctly discharged from the valve, the risk of LCO increases. In the extreme situation where the liquid fuel goes back in through the upper openings of the valve more quickly than it is purged through the drainage apertures, a situation may occur in which the float is blocked in the closed position. The valve therefore no longer makes it possible to ventilate the tank. Therefore it is generally necessary to find a compromise.
The idea behind the invention is to no longer have to make a compromise between the purge performance of the valve and the prevention of LCO, by creating a chamber in the base of the valve, the geometry of which is such that it makes it possible to prevent liquid fuel from following a direct path from the base of the valve to the top thereof (i.e., up to the ventilation aperture).
In order to do this, this chamber is provided, at its base, with at least one drainage aperture in staggered rows relative to (not aligned with) the drainage apertures of the housing in which the float slides, which effectively makes it possible to prevent the creation of a direct path for the ascending liquid fuel while retaining relatively large purge apertures in order to enable a rapid drainage of the valve housing and therefore a more effective ventilation.
Therefore, it is no longer necessary to find a drainage/LCO performance compromise. These two aspects can therefore be optimized simultaneously.
It should be noted that the expression “drainage aperture” is understood to mean an aperture having dimensions such that it can effectively discharge (drain) liquid. Typically, the area of such an aperture is greater than or equal to that of an aperture with a diameter of 2 mm, preferably greater than or equal to the area of an aperture with a diameter of 3 mm. Thus the apertures of a filter which have a diameter of less than 1 mm do not correspond to this definition. Document JP 2007/327417 describes a valve that has such a filter at the base of a chamber located underneath the housing in which the float slides, the objective of which is to stop the air bubbles that were entrained by the fuel during filling of the tank. Such a valve probably exhibits effective protection against the waves of liquid fuel that may give rise to LCO but, on the other hand, it is difficult to drain considering the size of the filter apertures. In fact, small holes protect against LCO on closure of the valve, but increase the risk of LCO on reopening of the valve.